<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<input type="text" id="inputId" />
		<button onclick="cal()">阶乘</button>
		=
		<span id="resultId"></span>
		
		<script type="text/javascript">
			function cal() {
				// var inputObj = document.getElementById('inputId');
				// var num = inputObj.value;
				var num = document.getElementById('inputId').value;
				var resultObj = document.getElementById('resultId');
				
				/* if (num == '') {
					resultObj.innerHTML = '<span style="color:red;">不能为空</span>';
					return;
				}
				
				if (isNaN(num)) {
					resultObj.innerHTML = '<span style="color:red;">您输入的不是一个数字</span>';
					return;
				} */
				if (num == '' || isNaN(num)) {
					resultObj.innerHTML = '<span style="color:red;">不能为空/您输入的不是一个数字</span>';
					return;
				}
				
				if(num == 0) {
					resultObj.innerHTML = 1;
					return;
				}
				
				// n!=n*(n-1)*(n-2)*..3*2*1
				var result = 1;
				for(var i = num; i >= 1; i--) {
					result = result * i;
				}
				resultObj.innerHTML = result;
				
			}
		</script>
	</body>
</html>
